1. Field of the Invention
This invention relates in general to earth boring drill bits, and in particular to the arrangement of the cutting elements.
2. Description of the Prior Art
The most common type of earth boring drill bits for oil and gas wells are cutters that rotate about an axis and roll around the bottom in a path or kerf as the bit rotates. The cutters have rows of teeth that disintegrate the earth formation through force applied on the cutter. The teeth are spaced in rows and spaced to disintegrate as much of the bottom as possible in a single rotation. The prior art earth drilling bits include various features designed to avoid a problem known as "tracking". This problem arises when the spacing of the teeth on a rotatable cutter enables the teeth to fall repetitively within previous tooth impressions in the earth. Eventually, ridges and peaks are formed in the earth, and as a result, the cutter experiences accelerated abrasive wear. The teeth are thus worn prematurely and unevenly. In bits with teeth of hard metal inserts retained by interference fit in drilled holes, the supporting metal may wear prematurely and the inserts may be lost.
Solutions to tracking are shown in U.S. Pat. No. 3,726,350, R.C.O. Pessier, Apr. 10, 1973, and in U.S. patent application Ser. No. 043,533, R.C.O. Pessier, filed May 29, 1979 now U.S. Pat. No. 4,316,515, issued Feb. 23, 1982 . Another solution is suggested in U.S. Pat. No. 4,187,922, F.E. Phelps, Feb. 12, 1980
In each of the above inventions, the inserts are arranged in circumferential rows, with varying spacing among inserts to prevent tracking. These prior art inserts are arranged in groups, with similar spacing in a group, but differing spacing in other groups; or the spacing in each row progresses from a minimum to a maximum and back to the minimum; or the insert spacing is varied in each row so that each pair of inserts is separated by a space different from the space between all other pairs of inserts in the row.
In each of the prior art solutions discussed above, the inserts are arranged in circumferential rows. The rows are separated by a minimum spacing to provide adequate supporting metal for the inserts. To prevent the generation of a ridge between rows, another cutter positioned in the same kerf or path may have staggered rows arranged to remove the earth where such ridges would otherwise form. Another method is to stagger the cutter itself from the other cutter in the kerf, such as shown in U.S. patent application Ser. No. 043,533, R.C.O. Pessier, filed May 29, 1979 now U.S. Pat. No. 4,316,515, issued Feb. 23, 1982. Occasionally, bits sometimes rotate "off-center", meaning that the rotational axis of the bit becomes displaced during drilling from the central axis of the borehole. One result of this phenomenon is the generation of ridges, even between staggered rows of the various cutters.
There are regions of prior art cutters which have annular rows that overlap without intervening spaces. In U.S. Pat. No. 3,726,350, the cutter has half rows offset from each other. E. A. Morlan disclosed in U.S. Pat. No. 2,774,571, Dec. 18,. 1956, the use of an inner end or "nose" of each cutter which has such an arrangement. J. H. Howard et al disclosed in U.S. Pat. No. 2,230.569, Feb. 4, 1941, a large number of arrangements for cutters with milled teeth, including helical rows of teeth. Also, shaft cutters with helical rows have been used in the prior art.
In all art known to applicant, the teeth or inserts are arranged in rows. The rows may be circumferential and perpendicular to the cutter axis, or the inserts in the row may only extend partially around the cutter. The rows may be parallel with the cutter axis, or the rows may be helical as mentioned. All of the various arrangements, however, cannot completely eliminate tracking and provide full coverage in a single kerf with a single cutter.